1. Field of the Invention
This invention relates to polycarbonates and more particularly to flame retarding agents for polycarbonates.
2. Description of the Prior Art
Polycarbonates derived from reactions involving aromatic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded products requiring impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties.
However, these polymers exhibit a brief though definite burning time when contacted with an open flame and do not meet certain specified requirements for flammability resistance in applications where high temperatures and/or exposure to fire may be encountered. In addition, stabilizers or other functional additives which are normally used in polycarbonates will further modify the burning characteristics of the plastic, to make them unacceptable where a certain flame retardance requirement is specified.
Also, stabilizers and functional additives such as monomeric phosphates, phosphoric acid esters and thiophosphoric acid esters containing halogenated alkyl radicals have been incorporated into polymers to increase their flame resistant properties. Metal salts have also been utilized to provide flame retardance characteristics to polycarbonates. Further, in aromatic polycarbonate resins some of the phenolic diols used in the production thereof have chlorine or bromine atoms substituted on the aromatic ring to provide flame resistant characteristics to the final polycarbonate.
Illustrative of the sulfonic acid salts and of metal salts are those disclosed in U.S. Pat. Nos. 3,775,367; 4,067,846; 4,073,768; 4,075,164; 4,066,618; 4,069,201; 4,093,589; 3,971,756; 3,933,734; 3,940,366, and 3,919,167, all incorporated herein by reference.
Some of these flame retardants, however, in order to be effective, are added in relatively large amounts such as to adversely effect some of the desirable properties of the base resin. For example, both the impact strength and the hydrolytic stability are compromised upon addition of large quantities of salt. Further, many of these flame retarding salts are susceptible to volatilization at the high molding temperatures of polycarbonates, necessitating thus the addition of excess amounts of salt which in turn bring about haze and loss of transparency. Since there is no uniformity of processing conditions among molders, it becomes difficult, if not impossible, to regulate the ultimate amount of salt incorporated into the polycarbonate.
Flame retardants incorporating a phthalimide group have been disclosed in, for instance, British Pat. No. 1,287,934 and U.S. Pat. Nos. 3,873,567; 3,923,734; 3,915,930; 3,868,388; 4,087,441; 4,001,179 and 4,003,862.
In accordance with the present invention a sulfonate salt is provided which is characterized by low volatility under normal polycarbonate processing conditions, which imparts improved flame resistance to polycarbonates even at low concentrations.